1. Technical Field
The subject invention relates to a method and apparatus for manufacturing a fracturable metal connecting rod, and more particularly it relates to a method and apparatus for manufacturing a flashless metal connecting rod of the type having a one-piece construction and being fracturable into a cap portion and a body portion, through a sequence of forging steps.
2. Discussion
Internal combustion engine connecting rods typically include a first end, or crank end and a second end, or pin end for interconnection of a crankshaft and a piston. The crank end forms a portion of an annular bearing assembly that requires a separable cap and body portions so as to permit insertion of a geometrically complex crankshaft. The pin end forms a portion of a pin bearing assembly for attachment to a piston.
Modern advancement of internal combustion engine technology has resulted in smaller engines which are able to deliver higher horsepower at increasingly higher rpms. Consequently, increased stresses are placed on an engine's connecting rods and its bearings. Such advancements have necessitated corresponding evolutionary changes to connecting rods to meet an increasing demand for lighter and stronger connecting rods.
Connecting rods for internal combustion engines are currently produced using one of three primary metal forming processes, each of which is associated with distinct advantages and disadvantages. The first such process involves "conventional forging" and includes separately forging attachable cap and body portions. In this process, a bar of hot rolled steel, aluminum, or other deformable metal is heated to a forging temperature of approximately 2200.degree.-2350.degree. F. and is placed between multiple cavity top and bottom forging dies in forging presses or hammers. An amount of metal in excess of that necessary to form the resultant product is placed between the dies to ensure complete filling thereof. The cap and body portions are formed through multiple impacts of the presses or hammers. The excess metal is generally directed between opposing dies, resulting in unfinished parts with a flashing surrounding each part at the die intersection.
In a subsequent operation, a set of trim dies is used in a trim press to punch the parts from the flashing. Extensive machining of the body and cap portions is then required which is not only cumbersome and expensive, but also does not ensure a perfectly matching of the cap and body portions under all operating forces.
Advantages associated with conventional steel forging of connecting rods include increased strength characteristics resulting from the natural grain flow of the rolled steel and grain improvement from the forging process. Additional advantages include high production rates, relatively inexpensive raw materials, and the ability to utilize multiple cavity dies. Disadvantages, however, with conventional steel forging processes include increased labor and expense associated with machining excess material from the forged part and the separate cap and body portions to match. The conventional forging process is also disadvantageous because it inherently results in inconsistent weights from part-to-part which necessitate additional machining to balance the final product.
The second method currently used for producing connecting rods is "casting" and involves heating a mixture of scrap metal, typically cast iron, in an electric furnace until it becomes molten. The molten metal is next poured into a "sprew" connected to tunnels or "runners" that permit the molten iron to flow into multiple cavity molds and vent through risers. After cooling of the metal, the iron castings are separated from the molds and the connecting "sprews" and "runners" are removed through machining.
The advantages associated with cast iron connecting rods include the relative low cost of iron, the absence of any secondary trim operation, increased tolerances, and utilization of multiple cavity molds. Disadvantages associated with cast iron connecting rods include lower strength characteristics compared to conventional forging, possible porosity defects, increased mold costs, and increased labor costs to remove the sprews, runners and other excess material. Additional disadvantages include increased costs due to the two part design of the cap and body portions and an absence of grain flow strength.
The third method of manufacturing connecting rods, "powder metal" involves placing a mixture of metal "powder" in a preformed die into a compacting press. The "powder" is compressed until it achieves a mechanical bond. The resulting preform is subsequently heat treated, or sintered, to achieve a chemical bond, thereby strengthening the preform. The perform is heated to a suitable forging temperature and then placed in a forging press which compresses the perform until the desired powder density and geometry is achieved. Excess material is extruded through vertical gaps between the top and bottom dies and is subsequently removed through mechanical grinding or trimming.
In a related "isothermal" powder metal process, the preform remains in the die through the sintering process. The die with the heated preform is transferred to a forging press where forging dies press the preform to specific dimensions independent of density.
Advantages associated with the powder metal forming techniques of manufacturing connecting rods include increased tolerances and thereby reduced machining costs, adequate compressive strength and ease in forming crank and pin holes with plugs. Other advantages include reduced waste material and uniform material composition of the resulting product. Disadvantages associated with the powder metal forming of connecting rods include the complexity of the process, the relatively high cost of "powdered" metal, relatively low tensile strength and impact strength, a potential for premature fatigue failure, and a lack of grain flow strength. Additional disadvantages include sizing and aperture tolerances which are dependent upon shrink variations which result from the sintering and forging processes, increased difficulties in machining the stronger powder compositions, and decreased density relative to other processes such as conventional steel forging.
The powder metal process of manufacturing connecting rods produces a single piece connecting rod having properties which permit the connecting rod to be fractured into a mating cap and body portions with an undulating interface. As is well known in the art, the undulating interface provides non-sliding surfaces where the cap and body portions are bolted together. Properly rejoined, the unique undulating interface prevents micro-shifting of the parts relative to one another and assures more accurate operating alignment. As a result, bearing life increases and the potential for premature failure of the connect rod decreases.
While those previously known methods for manufacturing connecting rods have proven to be commercially viable, each is associated with various above-noted disadvantages. The method and apparatus of the present invention provide advancements in precision forging technology to produce connecting rods which combine the collective major advantages associated with conventional steel forging, cast iron and powder metal connecting rods, without the disadvantages associated with prior art forging methods.